1. Field of the Application
This application relates to wireless communication techniques in general, and more specifically to symbol transmission in a MIMO scheme using Alamouti codes.
2. Background of the Disclosure
The demand for services in which data is delivered via a wireless connection has grown in recent years and is expected to continue to grow. Included are applications in which data is delivered via cellular mobile telephony or other mobile telephony, personal communications systems (PCS) and digital or high definition television (HDTV). Though the demand for these services is growing, the channel bandwidth over which the data may be delivered is limited. Therefore, it is desirable to deliver data at high speeds over this limited bandwidth in an efficient, as well as cost effective, manner.
A known approach for efficiently delivering high speed data over a channel is by using Orthogonal Frequency Division Multiplexing (OFDM). The high-speed data signals are divided into tens or hundreds of lower speed signals that are transmitted in parallel over respective frequencies within a radio frequency (RF) signal that are known as sub-carrier frequencies (“sub-carriers”). The frequency spectra of the sub-carriers overlap so that the spacing between them is minimized. The sub-carriers are also orthogonal to each other so that they are statistically independent and do not create crosstalk or otherwise interfere with each other. As a result, the channel bandwidth is used much more efficiently than in conventional single carrier transmission schemes such as AM/FM (amplitude or frequency modulation).
Space time transmit diversity (STTD) can achieve symbol level diversity which significantly improves link performance. STTD code is said to be ‘perfect’, therefore, in the sense that it achieves full space time coding rate (Space time coding rate=1, also called rate-1), and it is orthogonal. When the number of transmit antennas is more than 2, however, rate-1 orthogonal codes do not exist.
An approach to providing more efficient use of the channel bandwidth is to transmit the data using a base station having multiple antennas and then receive the transmitted data using a remote station having multiple receiving antennas, referred to as Multiple Input-Multiple Output (MIMO). MIMO technologies have been proposed for next generation wireless cellular systems, such as the third generation partnership project (3GPP) standards. Because multiple antennas are deployed in both transmitters and receivers, higher capacity or transmission rates can be achieved.
When using the MIMO systems to transmit packets, if a received packet has an error, the receiver may require re-transmission of the same packet. Systems are known that provide for packet symbols to be mapped differently than the original transmission.
Methods for transmitting symbols in a MIMO environment have been described in PCT International Patent Application no. PCT/CA2005/001976 bearing publication no. WO 2006/076787. This application is incorporated herein by reference.
In a closed loop system, the packet receiver can also indicate to the transmitter the best mapping of the re-transmit format.
In known systems, the possibility exists for certain symbol mappings to be ineffective in overcoming interference.
Thus a need exists for an improved ways to facilitate MIMO re-transmissions.